Composite bodies (multilayer bodies) in which various metals and resins are laminated and integrated have been used as exterior materials, protective materials, etc. for machines and buildings, in members such as parts for use in home appliances, mobile machines such as automobiles, general machinery, medical machinery, industrial machinery, building materials for buildings such as houses, and construction materials for civil engineering structures or the like. As an example of such multilayer composite bodies in which various alloys and resins are integrated, PTL 1 discloses a multilayer composite body of a metal and a resin suggested by the inventors of the present application et al., this multilayer composite body being obtained by treating a shaped aluminum alloy article with an aqueous hydrazine solution, a water-soluble amine compound, or the like, to obtain the shaped aluminum having the thus treated surface provided with an ultrafine uneven structure with a period of 20 nm to 50 nm, and then injection molding a resin such as a polybutylene terephthalate resin directly on the aluminum alloy to integrate the alloy and the resin.
Further, PTL 2 discloses a multilayer composite body obtained by chemically etching a surface of a metal to obtain an uneven structure on which granules or shaped irregular polygons with a diameter of 20 nm to 70 nm are laminated with an irregular period of 0.5 μm to 10 μm, and injection joining a polyphenylene sulfide resin, a polybutylene terephthalate resin, and a polyamide resin onto the metal. PTL 3 discloses a multilayer composite body obtained by coating a coat material of a thermosetting resin such as a urethane curable resin on a molded article of a hard plastic, and injection joining a polybutylene terephthalate resin onto the coated surface.
Meanwhile, polyolefin-based resins such as polypropylene resins have been widely used as inexpensive construction materials with excellent mechanical properties, and such resins generally excel in weather resistance, chemical resistance, and light weight; however, a problem arising because of the polyolefin-based resins being nonpolar resin compositions is that the adhesion thereof to metals is poor and multilayer composite bodies of metals and resins in which the polyolefin-based resins are joined to shaped metal articles cannot be easily produced.
To resolve this problem, it has been suggested to form a primer layer of a resin material including a polyolefin on the surface of a metal member and inject polypropylene, which excels in cost performance, onto the primer layer to join the metal member and polypropylene together (PTL 4).